The present invention relates to a method of producing pre-treated molten metal for casting objects.
The Swedish patent application 9304347-9 discloses a method of continuous and/or semi-continous casting of compacted graphite iron (CGI) wherein a structure modifying agent is added to the molten iron before it is supplied to a closed conditioning furnace provided with inlet and outlet channels, e.g a pressurized pouring furnace. In operation, the quantity of molten cast iron is maintained within predetermined limits by replacing, intermittently, the cast iron tapped from the conditioning furnace with a compensation amount of molten cast iron taken from a reaction vessel. Magnesium as structure modifying agent is added to the molten cast iron in the reaction vessel and, optionally, in the conditioning vessel. FIG. 1 discloses a preferred embodiment of this prior art.
The overpressure in the furnace space (16) can be regulated for controlling the melt flow through the tapping hole (13) placed in the spout (9). In case of longer stoppages the overpressure is removed so that the melt (5) in the spout (9) will be drawn back into the furnace (16) such that the melt level will be the same in the inlet and outlet channels as within the furnace chamber as indicated with broken lines in FIG. 1. Therewith, the fading rate of magnesium is reduced. The magnesium loss from the melt within the furnace chamber can be calculated and compensated by feeding Mg-wire (6) through the magnesium infeed valve (19) before restart of the operation.
However, the melt in the outlet channel will be subjected to relatively rapid magnesium losses due to evaporation and oxidation since the outlet is open to the atmosphere. In addition, the melt in the outlet channel cannot be re-charged with magnesium. As a result thereof, the first castings produced after the stop have to be scrapped as they will not have the desired structure.
From SE-328 673 (corresponding to U.S. Pat. No. 3,764,305) there is known a method to protect the melt in the outlet and inlet openings by flushing with inert gas. However, this method only reduces the magnesium losses to a limited extent and is not sufficient to produce good first CGI castings.